German laid-open print DE-A 42 03 748 discloses a method for utilizing and evaluating rechargeable smart cards wherein payments by means of a smart card are recorded in a nonvolatile memory of the card. At each charging, the preceding payment transactions are erased and the available credit balance stored. It is in particular provided that the most recently inputted or most recently debited value is indicated on a display located on the card. Such a method or device is intended to increase the acceptance of purse smart cards since the user has at least a certain possibility of checking his current credit balance or the most recent transaction.
Further, from German laid-open print DE-A 196 31 557 discloses a display unit for a smart card. Here, too, the display of the current credit balance is intended to increase the acceptance of purse smart cards. The presented smart card has a smart card processor with terminals as well as a magnetic stripe and a field for an embossing as is usual in cards. Further, a display controller is implemented as a separate unit for controlling the display. For the power supply required for the display, a solar cell is provided.
A smart card having a display and a solar cell or a battery is known from European laid-open print EP-A 0 299 414 as well as from U.S. Pat. Nos. 4,827,111, 4,924,075, and 5,438,679.
The disadvantage of known smart cards is that the power supply causes a problem on a smart card because of the low capacity of the available energy sources. In particular, it has proved problematic that due to the comparatively high clock frequency of the card microprocessor, which must be at least one megahertz for safety reasons, high current consumption considerably restricts the service life of the energy source or makes it very difficult or impossible to use a solar cell, for example, in smart cards.